I've been running in Bikilas for 1.25 years or so, and most of my running in the last 6 months has been hilly trails. I'm not sure if it's the gradual contraction/drying out of the trail surface, or the eventual wearing-out of my Bikilas, but I was really starting to notice the rocks; sharper ones were hurting a lot, and there was a lot of slipping in the mud, too.
I knew that the Spyridon is more recommended than the Bikila for these types of conditions, but I loved the second-skin feel of the Bikilas, and was concerned about the additional seaming (like for the tongue and laces, and in many other places there's additional reinforcement). I was afraid it would chafe – and it does, a bit, though not in any of the areas where reinforcement was conspicuous. I got my blisters at the 5-mile mark just north of my instep and at another inch farther north. Both of these places coincide with a harder material "ribbon" that seams to be intended to sturdy up the outsole.
But I've led with the cons; and some chafe seems par for the course with a new pair of shoes; I'm anticipating this will work itself out. For the FF-familiar runner moving into a more rugged model like the Spyridon, you may miss the socklike way that the other models feel barely-there. But if you run on variable surfaces and bumpy rocks, I think you'll never go back. Where the difference became obvious was how I'd sort of "brace" mentally for a bit of pain or for a slight height differential in the dirt as I landed, only to discover it was not really registered. These absolutely diffuse the impact of rocks.
The second thing I noticed, as here in Seattle summer just means warmer rain, was that as I skirted the edges of mud puddles, I felt the coolness and instinctively readied myself for the "between the toe ooze" that always happens, and subsequently, the wet foot. No big deal to me really--just a consequence of the Five Fingers shoe. I ran in the others all through the winter with constantly wet muddy feet, and just used it as motivation to run faster and harder so I'd stay warm. But with the Spyridon, mud is deflected; my feet didn't get remotely wet. There are rubber "bumpers" wrapping up the sides of the toe box to protect against this.
The sole is more grippy. Less slippy :) For me this added protection is well worth having a longer break-in period. I was very hesitant about all the reinforcements with this shoe, but the benefits really offset it. In fact, I'm sort of kicking myself (with my non-blistered foot) for not having upgraded sooner.
As for fit, I felt the toebox was a bit longer in these, though ultimately I didn't size down. I tried multiple pairs of the same model (and multiple colors) before I got the pair that fit best. There is LOTS of variation even within the same model and the same colorway; I really do recommend NOT buying Five Fingers shoes online; from pair to pair they deviate (apparently, as I was told by an REI salesperson, because they're handstitched).
If you're new to FF and heading off to trails or mountain walking or running, these will be a great transition shoe, and if you're upgrading to these for a smoother run and less distraction from rocks, water, and mud, I think you'll be very pleased with the insulation.